1. Field of the Invention
The present invention relates in general to a hydraulic pressure control apparatus and a hydraulically operated braking system for a motor vehicle, and more particularly to a hydraulic pressure control apparatus including a seating valve and a valve driving device operated with an electric current applied thereto to move a valve member of the seating valve, and a hydraulically operated braking system wherein the pressure of a working fluid in a wheel brake cylinder for braking a wheel of the motor vehicle is regulated by such a hydraulic pressure control apparatus.
2. Discussion of the Related Art
The assignee of the present application proposed a hydraulic pressure control apparatus including a seating valve and a valve driving device as described above, and as disclosed in Japanese Patent Application No. 8-17988 which had not been laid open at the time the above-identified Japanese Patent Application No. 9-88349 was filed. This hydraulic pressure control apparatus includes (1) a seating valve having a valve member and a valve seat and disposed in a fluid passage such that the fluid passage is separated by the seating valve into a high-pressure portion and a low-pressure portion when the seating valve is placed in a closed state thereof with the valve member being seated on the valve seat, and such that a difference between pressures of a working fluid in the high-pressure and low-pressure portions of the fluid passage acts on the valve member in a first direction for moving the valve member away from the valve seat so as to place the seating device in an open state thereof, (2) an elastic member for biasing the valve member in a second direction for moving the valve member to be seated on the valve seat, (3), a valve driving device operated with an electric current applied thereto, to generate a drive force acting on the valve member in the above-indicated first direction, and (4) a control device for controlling the electric current to be applied to the valve driving device, to thereby control the valve driving device.
In the hydraulic pressure control apparatus of the type described above, a biasing force based on an elastic force of the elastic member acts on the valve member in the direction for moving the valve member to be seated on the valve seat, while a biasing force based on the fluid pressure difference and the drive force or biasing force generated by the valve driving device act on the valve member in the direction for moving the valve member away from the valve seat. When the biasing force based on the elastic member is larger than a sum of the other two biasing forces, the seating valve is closed with the valve member being seated on the valve seat. When the former is smaller than the latter, the seating valve is open with the valve member being spaced apart from the valve seat. The biasing force generated by the valve driving device and acting on the valve member changes with the amount of the electric current to be applied to the valve driving device. Accordingly, it is possible to control the fluid pressure difference, namely, at least one of the fluid pressures on the high-pressure and low-pressure sides of the seating valve, by controlling the amount of the electric current to be applied to the valve driving device.
In a hydraulically operated braking system for a motor vehicle, for example, the seating valve of the hydraulic pressure control apparatus may be disposed in a fluid passage connecting a master cylinder and a wheel brake cylinder for braking a wheel of the vehicle. The master cylinder is adapted to pressurize the working fluid according to an operation of a brake operating member. The fluid pressure generated by the master cylinder can be reduced by a desired amount by the hydraulic pressure control apparatus, so that the reduced fluid pressure is applied to the wheel brake cylinder. Described more specifically, the fluid pressure in the wheel brake cylinder, which is lower than the master cylinder pressure, can be controlled by controlling the amount of the electric current to be applied to the valve driving device. Further, the seating valve of the hydraulic pressure control apparatus may also be disposed in a fluid passage connecting the wheel brake cylinder and a reservoir which is provided to accommodate the working fluid under the atmospheric pressure. In this case, the wheel brake cylinder pressure which is higher than the fluid pressure in the reservoir can be controlled by the hydraulic pressure control apparatus.
However, the hydraulic pressure control apparatus discussed above suffers from a problem that the operating response of the hydraulic pressure control apparatus is not sufficiently high upon initiation of its pressure control operation. That is, the control device of the hydraulic pressure control apparatus is arranged to initiate a gradual increase (from the zero value) of the electric current applied to the valve driving device, upon satisfaction of a predetermined condition for opening the seating valve, so that the seating valve is opened when the biasing force generated by the valve driving device based on the applied electric current has exceeded a difference of the elastic force of the elastic member from the biasing force based on the fluid pressure difference across the seating valve. This arrangement causes a time delay between the moment when the predetermined condition for opening the seating valve is satisfied, and the moment at which the seating valve is actually opened with the amount of the electric current exceeding a given critical point corresponding to the above-indicated difference between the elastic force and the biasing force based on the fluid pressure difference. Thus, the operating response of the hydraulic pressure control apparatus is not satisfactory upon initiation of its pressure control operation.